What should sites like facebook do about child pornography?

Of course, if found then law enforcement should be pulled in. However, what should facebook do (if antything) to prevent it?

There are multiple issues at stake. Should facebook...

examine every picture and post for questionable material? Personally, I do not want that as I do not trust them to determine what is questionable - they have already revealed themselves as suspiciously inadequate at defining hate speech and shutting that down. To me, this is similar to the phone company or the governemnt listening to every call in case a crime is being committed

On the other hand, they certainly should not let communities of deviants thrive, upload, and exchange child pornography, right?